Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode combinations).
As briefly discussed above, an external control device can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by manipulating controls on the external control device to modify the electrical stimulation provided by the neurostimulator system to the patient. Thus, in accordance with the stimulation parameters programmed by the external control device, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers appropriate stimulation energy to the volume of tissue that is targeted for therapeutic benefit (e.g., treatment of pain), while minimizing the volume of non-target tissue that is stimulated.
For example, in the context of SCS, to produce the feeling of paresthesia without inducing discomfort or involuntary motor movements within the patient, it is often desirable to preferentially stimulate nerve fibers in the dorsal column (DC) nerve fibers, which primarily include sensory nerve fibers, over nerve fibers in the dorsal root (DR) nerve fibers, which include both sensory nerve fibers and motor reflex nerve fibers. In order to stimulate the DC nerve fibers, while guarding against the stimulation of the DR nerve fibers, tripolar SCS systems may activate anodes that flank a single cathode in a medial-lateral electrical field, with the single cathode providing the stimulation energy for the DC fibers, while the flanking anodes guarding against the over-stimulation of the DR fibers, thereby increasing the therapeutic range of SCS for stimulating the desired DC fibers, while reducing the unwanted side effect of stimulating DR fibers (see J. J. Struijk and J. Holsheimer Tripolar Spinal Cord Stimulation: Theoretical Performance of a Dual Channel System, Medical and Biological Engineering and Computing, Vol. 34, No. 4, 1996, pp. 273-279; J. Holsheimer, B. Nuttin, G. King, W. Wesselink, J. Gybels, and P. de Sutter, Clinical Evaluation of Paresthesia Steering with a New System for Spinal Cord Stimulation, Neurosurgery, Vol. 42, No. 3, 1998, pp. 541-549).
However, the number of electrodes available, combined with the ability to generate a variety of complex stimulation pulses, presents a huge selection of stimulation parameter sets to the clinician or patient. For example, if the neurostimulation system to be programmed has an array of sixteen electrodes, millions of stimulation parameter sets may be available for programming into the neurostimulation system. Today, neurostimulation system may have up to thirty-two electrodes, thereby exponentially increasing the number of stimulation parameters sets available for programming.
To facilitate such selection, a patient care professional (e.g., a clinician, field engineer, sales representative, etc.) generally programs the neurostimulator through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC), personal data assistant (PDA), or other computerized device. The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the neurostimulator with the optimum stimulation parameter set or sets, which will typically be those that stimulate all of the target tissue in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated.
Thus, in the application of electrical neurostimulation therapy, the goal is to identify a pertinent paradigm of stimulation that properly stimulates neural tissue. To aid the patient care professional in correlating the paresthesia experienced by the patient during stimulation and the dermatomes corresponding the region or regions of pain experienced by the patient, computer programming systems typically include dermatome maps of the human body onto which regions of pain and regions of paresthesia experienced by the patient can be recorded to allow the patient care professional to determine the effectiveness of the therapy.
The computerized programming system may be operated by a clinician attending the patient in several scenarios.
For example, in order to achieve an effective result from SCS, the lead or leads must be placed in a location, such that the electrical stimulation will cause paresthesia. The paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the neurostimulator to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neurostimulator, with a set of stimulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the region of activation (ROA) or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the neurostimulator (typically by independently varying the stimulation energy on the electrodes), the region of activation (ROA) can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the region of activation (ROA) relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability.
One known computerized programming system for SCS is called the Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation. The Bionic Navigator® is a software package that operates on a suitable PC and allows clinicians to program stimulation parameters into an external handheld programmer (referred to as a remote control). Each set of stimulation parameters, including fractionalized current distribution to the electrodes (as percentage cathodic current, percentage anodic current, or off), may be stored in both the Bionic Navigator® and the remote control and combined into a stimulation program that can then be used to stimulate multiple regions within the patient.
Prior to creating the stimulation programs, the Bionic Navigator® may be operated by a clinician in a “manual mode” to manually select the percentage cathodic current and percentage anodic current flowing through the electrodes, or may be operated by the clinician in an “automated mode” to electrically “steer” the current along the implanted leads in real-time (e.g., using a joystick or joystick-like controls), thereby allowing the clinician to determine the most efficacious stimulation parameter sets that can then be stored and eventually combined into stimulation programs. In the context of SCS, current steering is typically either performed in a rostro-caudal direction (i.e., along the axis of the spinal cord) or a medial-lateral direction (i.e., perpendicular to the axis of the spinal cord).
In one novel current steering method, described in U.S. patent application Ser. No. 12/938,282, entitled “System and Method for Mapping Arbitrary Electric Fields to Pre-existing Lead Electrodes,” which is incorporated herein by reference, a stimulation target in the form of an ideal target pole (e.g., an ideal bipole or tripole) is defined and the stimulation parameters, including the fractionalized current values on each of the electrodes, are computationally determined in a manner that emulates these ideal target poles. It can be appreciated that current steering can be implemented by moving the ideal target poles about the leads, such that the appropriate fractionalized current values for the electrodes are computed for each of the various positions of the ideal target pole. As a result, the current steering can be implemented using an arbitrary number and arrangement of electrodes, thereby solving the afore-described problems.
Significantly, it may sometimes be desirable to estimate or predict the stimulation effects of electrical energy applied, or to be applied, to neural tissue adjacent to electrodes based on an estimation of the membrane response (e.g., transmembrane voltage potentials) of one or more neurons induced by the actually applied or potentially applied electrical energy. For example, given a specific set of stimulation parameters, it may be desired to estimate an ROA within the neural tissue of a patient based on an estimation of the neuronal response, which technique is described in U.S. Provisional Patent Application Ser. No. 61/427,059, entitled “Neurostimulation System For Implementing Model-Based Estimate of Neurostimulation Effects,” which is expressly incorporated herein by reference. This estimated ROA can then be displayed to the user for analysis to determine whether the tissue to be targeted for stimulation is likely to be therapeutically stimulated. The estimated region or regions of stimulation with the most effective therapy can then be stored for subsequent use. For example, if the stimulation region(s) is estimated and recorded during an OR mapping procedure, the stimulation region(s) can be recalled and displayed during a subsequent fitting procedure, in which case, they can be used as target region(s) to aid in optimum placement of an ideal target pole.
When estimating the neuronal response, and thus a region of stimulation, nerve fibers of fixed and uniform diameters are assumed. However, because the diameter of nerve fibers running through a spinal cord are not uniform (with the larger diameter nerve fibers being more excitable than smaller diameter nerve fibers), and in fact, may, on average, be substantially different from the diameter of the nerve fiber assumed by the algorithm used to estimate the neuronal response, the resulting estimation of the region of stimulation may be inaccurate. At the least, the display of the estimation region of stimulation may provide an incomplete picture of the region or regions of tissue that are stimulated. Furthermore, in the case where multiple ROAs are estimated and displayed, especially for subsequent use during a fitting procedure, it may be difficult to correlate the displayed ROAs with the corresponding pain regions to be treated.